606 PROCEEDINGB OF THE GEOLOGICAL SOCIETY. [June 20, 



shaped articular surface looks obliquely upwards and backwards, 

 giving, when the great length of the bone is considered (for it 

 reaches to nearly opposite the hinder cranial margin), an enormous 

 gape. The symphysial extremity is imperfect; but the arrangement 

 of the teeth is probably similar to that in the other genera of the 

 CrossoiDterygidce ; that is to say, a row of small conical teeth borders 

 the outer dentary margin. About the middle of the bone several 

 large distant teeth occur, having their sockets internal to the mar- 

 ginal row. These teeth are strong straight cones, plicate at the 

 base, surrounded at or above the middle by one or more rings of 

 short longitudinal furrows or pits. The inner or splenial margin 

 bears numerous rasp-hke teeth. The splenial and dentary margins 

 are thus separated by an interval,* considerable in front (where the 

 large teeth are socketed), but narrowing behind, and ceasing a little in 

 front of the articular end of the mandible. The outer surface of the 

 bone is covered with fine tubercles and less prominent connect- 

 ing ridges. The aspect is thus rather granular than reticulate. 



The vertebrae of WiombojDti/cJiius are osseous rings, two of which 

 measured respectively j^ and i-| of an inch in diameter, J^ and ^^- 

 in length. The area of the notochordal foramen is three-fourths of 

 the whole surface, greater, therefore, than in Strepsodus, but less 

 than in the typical annular vertebrae of Megaliclitliys and Rhizodopsis. 

 The spines associated with them on specimens in the British Mu- 

 seum consist of a cylindrical recurved shaft, an articular head with 

 convex inferior margin, and a compressed distal extremity*. 



Portions of jaws are found in the same beds with the remains just 

 described, whose outer surface is similarly ornamented, and whose 

 teeth are disposed in the same way. But these teeth are remarkable 

 for the great breadth of their bases as compared with the rapidity 

 with which they taper to a fine point, often incurved. 



Megalichthts, Agassiz. 



For the cranial and vertebral characters of this genus, and the 

 advance which the latter show upon the Saurodipterines with which 

 it is associated, I would refer to p. 12 of Prof. Huxley's Essay, 

 already quoted. 



The resemblance between Megcdiclitliys and HhomhojytycJiius is very 

 close, both in cranial structm-e and in the arrangement of the denti- 

 gerous bones. Mention has been made above of palatal tooth-bearing 

 plates. These are two in number, triangular, with rounded ante- 

 rior apex ; their posterior extremity is not seen, two inches being 

 the greatest length to which they are exposed. Each plate bears 



* In this and other genera it must be remembered that only those rertebrae 

 are spoken of which occur in actual association -n-ith characteristic portions of 

 the fishes to which they are ascribed. This caution is necessary, and, at the same 

 time, perhaps not always protective against error. For, on the one hand, there 

 are good grounds for suspecting in some genera a difference in dimensions and 

 even proportions of the vertebrre in the same colimm, and, on the other, the 

 presence of vertebrge belonging to more than one individual is possible on tlie 

 same block. The number of vertebrge still unaccoimted for is large ; but it would 

 be both premature and out of place to enter on their description here. 



